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Why masculinity is not something to roar about in the deer world
With their fearsome antlers, muscular bodies and impressive size, European red deer appear to be the epitome of masculinity. So why do the females of this species prefer stags with higher-pitched "feminine" calls?
This is the contradiction that is puzzling University of Sussex researchers. Dr David Reby and his colleagues Drs Ben Charlton and Karen McComb at the University's Mammal Vocal Communication and Cognition Research Group made the surprising discovery while playing male calls to red deer hinds that were at peak ovulation.
In general in the animal kingdom the deeper the male's call, the more attractive he is to females. The fundamental frequency, or "pitch", of the roars reflects the rate of vibration of the vocal folds in the larynx. A lower pitch is typically associated with high testosterone levels, maleness and dominance. In humans, it is interpreted as an indicator of physical or social dominance and masculinity, and in some hunter-gatherer populations it is an indicator of male reproductive success.
But during a study at INRA Experimental Deer Farm at Redon, in France, which involved red deer stag roars being played though two separate speakers, Dr Reby and his team noted that the females consistently went towards the speaker that played higher-pitched calls.
"Why females prefer more 'feminine' roars is still something of a mystery," says Dr Reby. "I use 'feminine' here in a dangerously anthropomorphic way - obviously hinds do not have a concept of femininity or masculinity, and it is likely that the roars do sound very male to them."
He says there are several possible explanations for the results. It could be that high-pitched calls are characteristic of more active and motivated males, or that they have specific propagation properties that give callers an advantage. It could also be that hinds have a perceptual preference for high pitched roars as they sound more like the calls of their calves and that males exploit this bias. In any case hinds mating with males with higher pitched roars probably benefit from having successful sons with high-pitched roars themselves. This hypothesis is supported by the facts that in free-ranging red deer, the pitch of roars appears to be an inherited trait, and males with higher pitched roars have a better reproductive success.
The findings, which are published online in the Proceedings of the Royal Society B on 28 April, 2010*, constitute the first evidence of a female preference based on the fundamental frequency of a male call in a non-human mammal - but crucially in the opposite direction to that normally assumed.
Dr Reby points out that the pitch of the call in other species of deer also seems to be related to their size in a way that is unexpected. The North American Elk, which is among world's largest species of deer, has a call that's more like a whistle, while the diminutive Corsican deer has one of the deepest roars.
Previous studies by the Mammal Vocal Communication and Cognition Research Group found that red deer hinds preferred stag calls that have low formant frequencies, which indicate larger males. The formants are the vocal tract resonances, which do not affect the pitch of the call but do give information about the size of the stag and about its chances of reproductive success.
Dr Reby says: "Our latest research results emphasise the need for further investigations of female preferences in mammals to understand the extreme variation of fundamental frequency values observed in male sexual calls."
Notes for editors
* 'Oestrous red deer hinds prefer male roars with higher fundamental frequencies', by David Reby, Benjamin D Charlton, Yann Locatelli and Karen McComb. Proceedings of the Royal Society B http://rspb.royalsocietypublishing.org/content/firstcite
To listen to deer calls,visit http://www.lifesci.sussex.ac.uk/cmvcr/Deer.html